Problem: Solve for $k$ : $-9 = 15 + k$
Answer: Subtract $15$ from both sides: $ \begin{eqnarray} \\ \hphantom{-9 {- 15}} &\hphantom{=}& \hphantom{k} \\ -9 &=& 15 + k \\ \\ {-15} && {-15} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -9 {- 15} &=& k \\ \end{eqnarray} $ Simplify. $ k = -24$